Referring to FIGS. 1-2, a cordless power tool, such a drill 100, typically has a housing 101, a motor M for driving a cutting tool, such as drill bit DB, and a power tool battery pack 10 electrically connected to motor M. Drill 100 may have a variable speed feature allowing the user to select the rotational speed of motor M. Typically such variable speed feature is integrated into trigger assembly TA, allowing the user to adjust the desired speed by pulling trigger assembly TA a certain distance.
A typical power tool battery pack 10 has a housing 13, cells 11 disposed in the housing 13 and connected between first and second terminals A, B, and a thermistor 12 disposed in the housing 13 and connected between first and third terminals A, C. Such power tool battery pack 10 can be connected to drill 100 via terminals A, B.
Typically trigger assembly TA has a first switch S1, a second switch S2 and a potentiometer R1. Accordingly, when the operator closes the trigger assembly, first switch S1 closes, thereby powering up integrated circuit IC1. Integrated circuit IC1 is a device such as a 555timer, micro controller, or other device capable of controlling the voltage or current or power control to motor M. After first switch S1 is closed, potentiometer R1 provides a voltage signal to integrated circuit IC1 relative to trigger position. As the position of potentiometer R1 varies, the voltage presented to integrated circuit IC1 varies. Integrated circuit IC1 responds by activating FET Q1 with on/off pulses allowing current to flow from terminal A through switch S1 through motor M through FET Q1 (when FET Q1 is directed to be on by integrated circuit IC1). By directing FET Q1 on and off with a variable duty cycle, motor speed can be controlled. As previously stated, as the trigger assembly TA is fully engaged, FET Q1 is bypassed by second switch S2 allowing full connection to the power tool battery pack 10 for maximum power capability.
It is preferable to ensure that certain power tools can only be used with certain battery packs, and to lock out undesirable battery packs, which may not meet quality requirements, etc.